Oil sands are sand deposits which in addition to sand comprise clays, connate-water and bitumen. Depending on the depth of the deposit, bitumen may be recovered by mining or in situ thermal methods. Recovering the highly viscous bitumen from the oil sand poses numerous challenges, particularly since large quantities of heat and water are required to extract the bitumen. Further, most oil sand deposits are located in remote areas (such as, for example, in northeastern Alberta, Canada), which can contribute to increased costs for transportation and processing, especially in harsh weather conditions. Because of these challenges, obtaining a good yield of bitumen product from the oil sands is desired in order to reduce costs and waste.
Oil sand ore in a mining and extraction operation is typically processed using mechanical means and chemicals addition to separate the bitumen from the sands. One of the most common extraction techniques is hot water extraction. Hot water, air and process aids are added to the oil sands, resulting in the formation of an oil-rich froth product that “floats” or rises to form a distinct hydrocarbon phase that can be separated from the aqueous layer. The waste (sand, clay, rock, bitumen, water and chemicals) after processing in combination with the spent processing water and chemicals from the plant are termed as tailings.
The physical and chemical properties of tailings are dependent on the ore body being mined, processing circuits employed and the chemicals/reagents used prior to disposal. Tailings can be disposed of or stored using a variety of different methods. The overall oil sands extraction process, due to its size of operation, creates a large volume of waste requiring complex disposal arrangements. Tailings can include high quantities of bitumen/hydrocarbon product that is not extracted during typical bitumen extraction process. It would be desirable to recover a significant portion of this valuable material rather than having it remain in the waste tailings.
Typically, naphtha is used to dilute the bitumen froth before separating the product bitumen by centrifugation. This process is called naphtha froth treatment (NFT). Other processes use a paraffinic solvent (for example, a mixture of iso-pentane and n-pentane) to dilute the froth before separating the product bitumen by gravity. This process is called paraffinic froth treatment (PFT). A portion of the asphaltenes in the bitumen is rejected by design in the PFT process and this rejection is an important component in achieving solid and water levels in the product bitumen that are significantly lower than those in the NFT process. The advantages of the PFT over the NFT are in the better bitumen product quality (lower solids and water) and potential lower costs, because of the elimination of the cost-intensive centrifuges being used in the NFT process.
The PFT process comprises three distinct units: froth separation unit (FSU), solvent recovery unit (SRU) and tailings solvent recovery unit (TSRU). The tailings from TSRU can comprise about 6 wt % hydrocarbon in the form of asphaltenes and maltenes mixed with solvent (n-pentane and iso-pentane). The solvent concentration in the asphaltenes-solvent mixture is typically less than about 1 wt %. The disposal of the tailings with the solvent-diluted asphaltenes affects the economics of the bitumen extraction process.
U.S. Pat. No. 7,357,857 (2008), to Baker Hughes, Inc., describes a method of extracting bitumen from a bitumen froth using a paraffinic solvent. The method requires mixing the froth with the solvent for a sufficient period of time to dissolve the solvent, then subjecting the mixture to gravity or centrifugal separation for a sufficient period of time to separate the water, solids and asphaltenes. A separation enhancing additive is present, such as a polymeric surfactant.
U.S. Pat. No. 5,968,349 (1999), to BHP Minerals Int'l, describes a process for the extraction of bitumen from bitumen froth using a counter-current decantation circuit with a paraffinic solvent.
US Application 2005/0197267, published Sep. 8, 2005 in the name of Troxler Electronics Laboratories, Inc., describes water-soluble solvent compositions for removing petroleum residue from a substrate. The compositions comprise an aromatic ester, a cyclic terpene or a terpenoid, an odor-masking agent and a nonionic surfactant. The method contemplates using a spinning band distillation column.
US Application 2006/0196812, published Sep. 7, 2006, describes a method for diluting a bitumen froth with naphtha and contacting it with a zone settling aid such as a polyoxyalkylate block polymer.
Canadian Patent Application No. 2,645,450, published Sep. 13, 2007 in the name of Western Oil Sands (USA), Inc., describes a method of recovering asphaltenes from asphaltene-containing tailings using flotation separation and hydrophobic agglomeration separation.
Other methods and systems are described in the art, such as in U.S. Pat. No. 7,566,394 (Saudi Arabian Oil Company, 2009), US Patent Application No. 2008/0006561 (Moran et al., 2008), US Patent Application No. 2007/0295640 (Schlumberger Technology Corp., 2007), and Canadian Patent No. 2,614,669 (Imperial Oil Resources Limited, 2008).
There is a need to improve the reclaiming of the valuable hydrocarbon material from the tailings waste streams generated in a PFT process. There is also a need to recover the valuable hydrocarbon material from the tailings of the froth flotation step of the bitumen extraction process.